144 THE STRUCTURE AND LIFE OF BIRDS chap. 



present one under the head of " Hollow Bones." 

 For what remains the reader is referred to the re- 

 marks on " Passive Machinery " in the next chapter. 



Ligaments. 



Ligaments are like tendons in having no power of 

 contraction, but, unlike tendons, they are not con- 

 nected with muscles. Their usual function is to 

 fasten two bones together at the joint, and to limit 

 the amount of freedom with which one turns upon 

 the other. When a skeleton is obtained by macera- 

 tion — i.e., by leaving the carcase in water till the flesh 

 is easily removable, many of the ligaments still re- 

 main and keep the bones in their proper connection. 

 There are some which answer very different purposes. 

 The horse's head is supported by a strong elastic liga- 

 ment attached to the upright spines of the vertebrae. 

 The bird, as I shall show in the article on " Passive 

 Machinery" in the next chapter, has several which 

 are remarkable for their elasticity, some, if not all, of 

 these having been originally tendons. 



Feathers — Structure and Development. 



A feather is a very elaborate appendage. When 

 we are told that a Peacock's or an Ostrich's plume, or 

 the wing-feather of an Albatross is an " epidermic 

 growth," part, that is, of the horny outer skin, we 

 seem to hear words that explain nothing. There is 

 another " epidermic growth," the nature of which it is 

 perhaps hardly less difficult to realise — the horn of a 



